What kind of license do you have on your original legit Vista OS? If you still have a legit key, all you need to do is find somebody who's got the same version and get him/her to run you off a backup copy of the disk. If you want to go through the "official" channels, you should also be able to buy replacement media from MS to use your legit key on.

There are also ways to get a different version of Vista to recognise your key (assuming it's legit), but you'll have to google that one yourself as it's getting a bit too close to the line WRT the EULA for a discussion here.